Teacher S Version

Senior Form Chemistry
– Atomic theory /

Atomic Theory

Teacher’s Version

NOS/STSE Features
1. Laws describe relationships among observable phenomena. Theories are the explanations of these descriptions.
2. There is no hierarchical relationship between a law and a theory.
3. Scientists often collaborate.
4. Tentative nature of science.

Table of Contents

Introduction / (i)
Activity 1: Conservation of mass / P.1
Activity 2: Law of Definite Proportions / P.4
Activity 3: Atomic Theory / P.6
Activity 4: Assessment / P.10
Appendix 1: Activity Guide / P.13
Appendix 2: Lesson Plan / P.14

Senior Form Chemistry
– Atomic theory /

Introduction

Rationale

1. While students may not have much difficulty in reciting or stating the Law of Conservation of Mass, extensive research on students’ understanding has demonstrated that many of them are not able to apply the law in new situations or to think with the aid of the law. While we frequently refer to the Law of Conservation of Mass in our teaching of chemistry, surprisingly the law is not addressed in the chemistry curriculum or textbooks explicitly. It is believed that if students learn it consciously and through a variety of activities, they can have a stronger foundation for learning other chemistry concepts. For the teaching or learning activities about the law, please see Activity One and the Assessment Tasks.

2. Performance of Hong Kong (HK) Form 2 students in international comparative studies of science achievement shows that they are not very capable of drawing conclusions from experiment results. For example this question from the Trends in International Mathematics and Science Study (TIMSS) conducted in 1999.

Two open bottles, one filled with vinegar and the other with olive oil, were left on a window sill in the sun. Several days later it was observed that the bottles were no longer full. What can be concluded from this observation?

(A) Vinegar evaporates faster than olive oil. 6%

(B) Olive oil evaporates faster than vinegar. 3%

(D) Only liquids containing water evaporate. 17%

(E) Direct sunlight is needed for evaporation. 25%

Less than half of the HK students were able to choose the correct answer. Although drawing a conclusion from data is part of our science curriculum objectives, it is suspected that students either (i) sometimes take their ‘experiment result’ as the ‘conclusion’ or (ii) do not have sufficient training in this aspect. In an attempt to tackle this, please see Activity Two.

3. Many people believe that a ‘law’ states knowledge that has been “proven” while a ‘theory’ is something that has not yet been confirmed. In fact, generally speaking, a ‘law’ is a generalized statement about the empirical world; a ‘theory’ can be regarded as an explanation of some phenomena. For example, the pressure law states that, in a fixed mass and volume of gas, the temperature of the gas sample varies in direct proportion to its pressure. It should be noted that the pressure law (i) is not absolute – the behaviour of real gases can deviate from it; and (ii) does not explain why gases, real or ideal, behave like this anyway. To make sense of the properties of gas, the kinetic theory of gases was put forward. From this example, we do not see any hierarchical relationships between ‘law’ and ‘theory’. They are simply complementary in helping us to better understand nature.

In most instances, atomic theory is presented to students without addressing the scientific context concerning John Dalton who proposed the theory in 1808. A historical analysis reveals that the theory was an attempt to explain the ‘Law of Conservation of Mass’ (1774, Antoine Lavoisier) (Please refer to Activity One) and the ‘Law of Definite Proportions’ (1774, Joseph Proust) (Please refer to Activity Two). It is hoped that, through this package, students can understand (i) how science can be developed through the emergence of laws and subsequently, overarching theories that explain many physical phenomena; (ii) a possible relationship between a law and a theory; (iii) significant scientific knowledge can develop not only from direct experimental findings or observation. As we have seen, the two laws offer no direct evidence or proof of the existence of atoms. Dalton had not seen atoms. (Lavoisier had openly spoken against atomic theory, because he had no knowledge of atoms from experiments.)

Level of study Number of lessons

Form 3 – Form 6 3 lessons (each lesson 35-40 minutes)

Contents

1. Introducing the Law of Conservation of Mass

2. Discovery of the Law of Definite Proportions

3. Atomic theory as the explanation to the laws.

4. Assessment of students’ learning

Teaching package includes

1. Worksheets (Teacher’s Version and Student’s Version)

2. A Teaching PowerPoint

(ii)

Senior Form Chemistry
– Atomic theory /
Activity 1
Conservation of mass

If a piece of magnesium is burnt, will there be a gain or a loss in mass? Think about it!

Remarks: The setup is weighed before and after burning. (See video 2)

Results of investigation:

Mass of set up before burning = 26.093 g

Mass of set up after burning = 26.141 g

Change in mass = + 0.048 g

This is not a closed system. There will be a gain or loss of mass. In this case, the gain in mass results from the reactions of oxygen or nitrogen with magnesium.

Now divide into groups and conduct one of the following tasks to see whether there are changes of mass. After the task, present your findings to your classmates.

Notes: The aim of all these activities is to bring out the concept of the conservation of mass, but not the name of the chemicals or any explanation of the reaction.

What can you conclude from the above investigations?

In none of the cases does the total mass change after any reaction / process. (Conservation of mass)
[Notes: Teachers would collect students’ findings and bring out the Law of conservation of mass which was put forward in 1774, ‘In a chemical reaction, the mass of the products is equal to the mass of the reactants.’ In other words, ‘Matter can neither be created nor destroyed in any physical or chemical change.’ (A nuclear change also obeys the law of conservation of mass. In this kind of reaction, the mass change is proportional to the energy to the energy change i.e. e=mc2.)]

Notes: Teachers need to highlight to students that it was a great discovery. People before believed that matter can be created or could disappear because of human activity, however, people had no idea why the law was always true, even after it was put forward.

Extension activity

The Law of Conservation of Mass was put forward by Antoine Lavoisier in 1774. Lavoisier is known as the father of the modern chemistry. Find out more about his investigations!
Find evidence to answer the following questions in your project:
What were the major discoveries by Lavoisier?
How did he demonstrate the Law of Conservation of Mass?
Did he work alone in all his discoveries?
(Notes: This activity can be considered as a group project. Students may report their findings by various methods such as PowerPoint, poster, report, etc.)
You can search for information from libraries or the internet. Here are three references:
1. http://www.pbs.org/wgbh/nova/einstein/ance-m.html(English only)
2. http://www.bud.org.tw/museum/s_star20.htm (Chinese only)
3. 多諾萬 (1997) 拉瓦謝: 化學改革與法國革命的先鋒。台北: 牛頓。
Activity 2
Law of Definite Proportions

At the end of 1700s, scientists were trying very hard to discover laws or “regularities” in Nature. Some chemists tried to mix different elements to see how they reacted with each other. They could measure the mass of the reactants and products very accurately.

Here are some data, or experimental results, they obtained from the reaction of carbon with oxygen to form carbon dioxide.

Mass of oxygen reacting (g) / Mass of carbon reacting (g)
32 / 12
64 / 24
96 / 36

Under some other conditions, carbon reacts differently with oxygen to produce carbon monoxide. Here are the data for these reactions:

Mass of oxygen reacting (g) / Mass of carbon reacting (g)
16 / 12
32 / 24
48 / 36

Here are the data for the reaction between copper and oxygen:

Mass of oxygen reacting (g) / Mass of copper reacting (g)
16 / 64
32 / 127
48 / 191

Remarks: The data is not the original data found by scientists in 1700s. Carbon and oxygen are used to make up a simple case for students. However, students are experiencing the same process as what scientists did in the past, i.e. to find patterns in the nature.

Can you discover any patterns in these data? Use your own words, draw a conclusion from these experimental results.

Suggested answers:
· There is a constant ratio between the mass of two elements which, on reaction, produces their products/ the compound.
· There is a constant ratio between the mass of oxygen and the mass of carbon which produces carbon monoxide (CO).
· There is a constant ratio between the mass of oxygen and the mass of carbon which produces carbon dioxide (CO2).
· There is a constant ratio between the mass of oxygen and the mass of copper which produces copper(II) oxide (CuO).

Notes: After the activity, the teacher can bring out the law of definite proportions, ‘A given chemical compound always contains the same proportion by mass of its constituent elements.’ The law was proposed by Joseph Louis Proust in 1797. Again, this was a great discovery at that time. However, nobody could explain why elements behaved in this way.

Activity 3
Atomic Theory

The Law of Conservation of Mass and the Law of Definite Proportion were great discoveries. At that time, nobody could explain the laws. However, in about 1808, John Dalton proposed the Atomic Theory which can be summarized by four points.

(a) All matter is composed of very small discrete particles.

(b) For elements, the particles are called atoms and are indivisible and indestructible in chemical reactions.

(c) (i) Each atom of a given element has the same mass. (ii) All atoms of a given element are identical. (iii) The masses of atoms of different elements are different.

(d) A chemical compound is formed from its elements by the combination of the different atoms in a fixed ratio for that compound.

How can this theory explain the Law of Conservation of Mass and the Law of Definite Proportions with reference to activities 1 and 2? Discuss with your classmates.

(a) & (b) Relationship to the Law of Conservation of Mass: if atoms could be destroyed, the mass of the products in Activity 1 should be less than that of the reactants.
(c) If atoms of different elements are the same, the mass of oxygen should be a simple multiple of the mass of carbon.
[Probing questions: (1) From the ratio you obtained in Activity 2, do you think the atoms of oxygen, carbon and copper have the same mass? Explain. (2) From the ratio, can you predict which atom, oxygen, carbon or copper, has the greatest mass?]
(d) In carbon monoxide, one carbon atom joins to one oxygen atom – 1:1. In carbon dioxide, one carbon atom joins to two oxygen atoms – 1:2. This illustrates the fixed ratio.
[Probing question: Use your imagination, try to use different kinds of circles to represent different kinds of atoms and draw the products, carbon monoxide and carbon dioxide.]

What are the differences between a law and a theory?

Laws describe relationships among observable phenomena.
Theories are the explanations of those statements.

In your daily lives, you may be familiar with the law below:

Does the law explain the behavior of a material (e.g. a gas) in different temperature? Explain your answer.

A law only describes natural phenomena but does not explain them. In this case, the law describes when the material would expand or contract. Scientists proposed the law to generalize the behaviors of materials at different temperaturess. We can use the law to predict the behaviors of certain material under different condition but the law itself gives no explanations of these behaviors. It is the kinetic theory which explains the behaviors.
(Notes: This question aims to assess students’ understanding of the definition of law. Students might get the wrong answer and teacher can further discuss the differences between laws and theories with students.)

Notes: For higher form students, teacher may use the ideal gas law (PV = nRT) as a substitute for the law of “thermal expansion and contraction”.

You may have also learnt:

Is it a law or a theory? Explain.

A law (Law of reflection). It describes the phenomenon that the angle of reflection of a light ray is always equal to its angle of incidence at a flat, smooth reflective surface.

Can you give some others examples of laws and theories that you have learnt?

Examples:
Law:
Newton’s Laws
Mendel’s Laws of Genetics
Theory:
Particle theory of light
Theory of relativity
Genetic theory

Remarks:

The following are different “definitions” of “law” and “theory”:

Laws are generalizations, principles or patterns in nature. (McComas, 1998)